Method and apparatus for suppressing power frequency common mode interference

ABSTRACT

An apparatus for suppressing power frequency common mode interference in a bioelectrical signal measuring system includes a driving circuit configured to amplify and change the phase of a common mode interference signal to produce a first driving signal, wherein the common mode interference signal is extracted from a plurality of first electrodes attached to a patient; phase compensating and processing circuitry electrically connected to the driving circuit for receiving the first driving signal, the phase compensating and processing circuitry configured to produce a second driving signal by phase-compensating the first driving signal based on a characteristic value of power frequency interference in a bioelectrical signal acquired through the plurality of first electrodes; and a switch to receive the first driving signal and the second driving signal, the switch configured to selectively switch between providing the first driving signal and providing the second driving signal to a second electrode attached to the patient.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/316,190, filed Dec. 22, 2005, now U.S. Pat. No. 7,684,853, whichclaims the benefit of Chinese Patent Application No. 200510100533.8,filed Oct. 19, 2005, both of which are incorporated herein by referencein their entireties.

FIELD OF THE INVENTION

The present invention relates to an anti-interference technique used indetection of bioelectrical signals, more specifically, to an improvementand a method of a driving circuit for suppressing power frequency commonmode interference.

DESCRIPTION OF THE RELATED ART

Power frequency (industrial frequency) interference widely exists duringthe measurement of the bioelectrical signals, and is generally caused byindoor illumination or power equipments. The interference frequenciesare various, generally 50 Hz or 60 Hz, depending on the frequencies ofthe power grid in different countries. Since such frequencies normallyfall into the frequency range of the bioelectrical signals themselves,such as electrocardiograph (ECG) signal and electroencephalogram (EEG)signal, it is critically important to improve the anti-interferencecapability of the equipment for acquiring high-quality bioelectricalsignals.

Presently, a driving circuit as shown in FIG. 1 is widely employed inthe power frequency interference suppressing technique adopted in themeasurement of the bioelectrical signals. For example, the circuit isconnected but not limited to a right leg of a human body. The circuitincludes an auxiliary amplifier A₃, an output terminal of which isconnected to the right leg via a resistor R₀, and an input terminal ofwhich receives power frequency interference common mode voltage from thehuman body (for example, in the ECG detection shown in FIG. 1 a,amplifiers A₁ and A₂ acquire a pair of differential detection signalfrom the human body, and the common mode voltage can be outputted fromthe series connection point of the two resistors connected in seriesbetween the differential signals). Since the right leg is not in contactwith the ground directly, the displacement current of the human bodywill flow to the resistor R₀ and the output terminal of the auxiliaryamplifier. Here, the resistor R₀ has a protecting function, that is,once a high voltage exists between the patient and the ground, theauxiliary amplifier A₃ is in the saturation state which is equivalent asconnecting the ground, and the resistor R₀ perform a function oflimiting current to protect the human body.

The common mode voltage upon the auxiliary amplifier being in thesaturation state may be calculated from an equivalent circuit of theright leg driving circuit shown in FIG. 1 b. Let the common mode gain ofthe high impedance input stage is 1 and the voltage of the outputterminal of the auxiliary amplifier is V₀, then in the invertingterminal of the auxiliary amplifier

$\begin{matrix}{{\frac{2V_{cm}}{R_{a}} + \frac{V_{0}}{R_{F}}} = 0} & ( {2\text{-}1} )\end{matrix}$and then

$\begin{matrix}{V_{0} = {- \frac{2R_{F}V_{cm}}{R_{a}}}} & ( {2\text{-}2} )\end{matrix}$

Since V_(cm)=I_(d)R_(o)+V_(o), then

$\begin{matrix}{V_{cm} = \frac{I_{d}R_{0}}{1 + \frac{2R_{F}}{R_{a}}}} & ( {2\text{-}3} )\end{matrix}$

Thus, |V_(cm)| can be minimized by both using the right leg drivingcircuit and increasing

However, in a practical ECG signal measurement environment, a low-passfilter is normally added in the input stage of the driving circuit forfiltering out high-frequency interference. At the same time, the actualequivalent circuit of the right leg driving circuit is the circuit shownin FIG. 2, since there are electrostatic coupling capacitances existingamong the human body, measuring instruments and power lines. In theequivalent circuit, C_(s) is distributed capacitance between a commonmode amplifier and the ground, C_(b) is distributed capacitance betweenthe human body and the ground, G is an inverting amplifying factor,R_(ei) is electrode impedance, and R_(o) is a current limiting resistor.A phase shift of 180° (wherein, “H” shown in FIG. 2 denotes a transferfunction,

$H = \frac{1}{1 + {\frac{G}{2\pi\; B}s}}$is introduced in the closed-loop system.

Amplifiers A1 and A2 achieve unity gain, and do not produce phase shiftsin interested frequency domains. However, the auxiliary amplifier A3 hasa pole at the frequency f=B/G (wherein B is a gain bandwidth).Meanwhile, the low-pass filter formed by a second order RC network maychange the phase shift of the system (the figure ignores the electrodecapacitance). As no way to determine R_(ei) and C_(b), the accurateposition of the pole cannot be determined, such that the pole cancelingis difficult to achieve. In order to ensure the stability of the system,the measures of lowering a corner frequency and adding a feedbackcapacitance are usually employed. However, the output signal of theright leg driving and amplifying circuit is not in opposite phase withthe common mode signal completely due to the effect of the feedbackcapacitance.

Take an application circuit of the right leg driving circuit shown inFIG. 3 as an example, wherein its frequency characteristic is shown inFIG. 4. It can be seen that the phase of the feedback signal outputtedfrom the circuit is not in opposite phase by 180°, but by 126° at theinterference frequency of 50 Hz. Therefore, the actual effect ofsuppressing power frequency common mode interference cannot reach thelevel of the theoretical analysis. Furthermore, the circuit delay of theactual measuring apparatus is affected by the parameters of circuitcomponents, operational amplifiers and cables. In the process ofmeasurement, the human body, measurement environment and measuringapparatus constitute a complicated measuring system, and the impedanceof the human body, skin electrode impedance and electrostatic couplingcapacitance will make the measuring-system transfer function morecomplicated, wherein the human body is in the important section withinthe feedback circuit, thus the parameters of the human body becomecritical factors to the changing of the system transfer function.However, since the individual differences among human bodies areunpredictable, the anti-interference capability of the ECG measuringsystem is of indeterminacy.

In conclusion, the disadvantages of the above-mentioned relatedtechnique are the phase of the feedback signal outputted by the drivingcircuit is unable to reach the complete opposite phase upon measuringbioelectrical signals each time, because the frequency characteristic ofthe hardware circuit is constant, while the system transfer function isuncertain upon practical measurements, thus the system capability ofsuppressing power frequency interference is reduced.

SUMMARY OF THE INVENTION

In view of above-mentioned disadvantages of prior art, the object ofthis invention is to provide a method and apparatus for suppressing thepower frequency common mode interference, which is used in abioelectrical signal measuring system. By improving the driving circuitthrough auto-performing phase compensation to make the amplified andoutputted feedback signal in opposite phase by 180° with the powerfrequency common mode interference signal, the system capability ofsuppressing power frequency common mode interference is thus gettingimproved.

To resolve the above technical problems, the basic idea of thisinvention is to provide an embedded system circuit in the drivingcircuit (for example, the driving circuit is connected with a right leg,but not limited to the right leg), in which it receives an amplifiedsignal originally outputted by the driving circuit, collects abioelectrical signal, and further analyzes the bioelectrical signal todetermine the auto-compensation amount of the phase of the originalamplified signal, so the compensated amplified signal is able tofeedback to the right leg for offsetting the power frequency common modeinterference signal. By this method the capability of suppressinginterference obtains improvement and facilitates the measuring system tocollect the high qualified ECG signal. Besides, if providing a switchingand selecting means, the embedded system circuit for compensating phasesis getting further controlled as to decide whether utilize it or not.

In the first aspect of the invention, an apparatus for suppressing powerfrequency common mode interference and used in a bioelectrical signalmeasuring system is provided, which comprises: a common modeinterference signal extracting circuit; and a driving circuit connectedto the extracting circuit, wherein the driving circuit changes a phaseof a common mode interference signal and amplifies the common modeinterference signal, so as to output two-way amplified signals, and oneamplified signal is selectively outputted to a living body onexamination. More particularly, the apparatus further includes phasecompensating and processing means for receiving both a bioelectricalsignal from the living body on examination and the other amplifiedsignal outputted from the driving circuit, determining a phasecompensation amount of the other amplified signal outputted from thedriving circuit according to a characteristic value of the powerfrequency interference in the bioelectrical signal so as tophase-compensate the other amplified signal outputted from the drivingcircuit, and selectively outputting the phase-compensated amplifiedsignal to the living body on examination.

In the above-described solution, the phase compensating and processingmeans includes: at least one A/D converter for converting thebioelectrical signal and the other amplified signal outputted from thedriving circuit into digital signals respectively and supplying thedigital signals to a microprocessor, which determines the phasecompensation amount of the other amplified signal outputted from thedriving circuit according to the characteristic value of the powerfrequency interference in the bioelectrical signal; and a D/A converterfor receiving a signal from the microprocessor and converting the signalinto an analog signal.

In the above-described solution, the apparatus further includes alow-pass filter, through which the phase-compensated amplified signaloutputted by the phase compensating and processing means passes beforebeing transmitted to the living body on examination.

In the above-described solution, the apparatus further includesswitching and selecting means with two selecting terminals, one of whichis connected to an output terminal of the driving circuit, the other ofwhich is connected to an output terminal of the phase compensating andprocessing means, and the switching and selecting means selectivelyoutputs the amplified signal outputted by the driving circuit and thephase-compensated amplified signal outputted by the phase compensatingand processing means to the living body on examination.

In the above-described solution, the apparatus further includesswitching and selecting means with two selecting terminals, one of whichis connected to an output terminal of the driving circuit, the other ofwhich is connected to an output terminal of the low-pass filter, and theswitching and selecting means selectively outputs the amplified signaloutputted by the driving circuit and the phase-compensated amplifiedsignal outputted by the phase compensating and processing means to theliving body on examination.

In the second aspect of this invention, a method for suppressing powerfrequency common mode interference is provided, which is used forsuppressing interference of power frequency common mode signal during abioelectrical signal measuring system detects bioelectrical signals,wherein the measuring system comprises a common mode interference signalextracting circuit and a driving circuit connected to the extractingcircuit for both changing phases of a common mode interference signaland amplifying the common mode interference signal, characterized inthat the method includes steps of:

A. providing phase compensating and processing means between the drivingcircuit and a living body on examination;

B. receiving an amplified signal outputted from the driving circuit anda bioelectrical signal from the living body on examination by the phasecompensating and processing means;

C. analyzing the characteristic of the bioelectrical signal anddetermining a phase compensation amount of the amplified signaloutputted from the driving circuit by the phase compensating andprocessing means, wherein the characteristic of the bioelectrical signalis represented by a characteristic value of the power frequencyinterference in the bioelectrical signal;

D. performing a corresponding time delay processing on the amplifiedsignal outputted by the driving circuit;

E. selectively transmitting the delay signal to the living body onexamination.

In the above-described solution, the step C includes steps of:

C1) initializing a system state value of the measuring system;

C2) extracting a characteristic value of the power frequencyinterference in a current system state of the measuring system;

C3) adding 1 to the system state value and extracting anothercharacteristic value again;

C4) if the another characteristic value being decreased, it indicatesthe power frequency interference is reduced, thus the direction of thecompensation is correct, then continuing the step C3) till the systemstate value becomes maximized; otherwise, it indicates the direction ofthe compensation is incorrect, then subtracting 1 from the system statevalue, and then an optimum system state value can be selected, whereinthe system state value is integer and corresponds to one phasecompensation state;

C5) setting the optimum system state value in the step 4) as a statevalue of the current system, to determine a corresponding phasecompensation amount; the above steps C1)˜C5) can be performedcircularly.

In the above-described solution, the system state value ranges from [0,K], in which

${K = {{{int}( \frac{f_{s}}{200} )} - 1}},$int( ) denotes rounding operation, and f_(s) denotes a samplingfrequency of the bioelectrical signal.

In the above-described solution, the characteristic value of the powerfrequency interference refers to a sum of peak to peak values of thepower frequency interference signal extracted from the bioelectricsignal within a plurality of periods, which is calculated by steps of:

a) successively storing data of the bioelectrical signal sampled by theA/D converter into a predetermined data array;

b) receiving the data array by a digital band-pass filter so as toextract the power frequency interference signal and output related data;

c) detecting a maximum value and a minimum value of the output datawithin a period, so as to calculate the peak to peak value of the signalwithin the period;

d) calculating a sum of the peak to peak values in a plurality ofadjacent periods so as to get the characteristic value of the powerfrequency interference.

In the above-described solution, the step D for performing acorresponding time delay processing on the amplified signal outputted bythe driving circuit includes steps of:

D1) controlling the A/D converter by a microprocessor of the phasecompensating and processing means, setting a sampling channel and asampling frequency f_(s), and sampling the amplified signal outputted bythe driving circuit;

D2) creating a data array org_data[K+1], and successively storingamplified signal sampled by the A/D converter;

D3) determining a delay-output datum according to an optimum state valueJ of the current system, which corresponds an array elementorg_data[K−J] of the data array;

D4) converting the datum outputted at the sampling frequency f_(s) intoan analog signal by the D/A converter and then outputting the analogsignal;

wherein

${K = {{{int}( \frac{f_{s}}{200} )} - 1}},$int( ) denotes a rounding operation.

In the above-described solution, in the step D, the signal outputted bythe driving circuit is transmitted to a delayer for performing acorresponding time delay processing, which is controlled by amicroprocessor and whose delay time is adjustable.

With the above-described technical solutions, the automatic phasecompensation of the driving circuit during the process of themeasurement of the bioelectrical signals can be achieved by takingadvantages of the characteristics of the close combination between thesoftware and the hardware of the embedded system, as well as thestability of the analog circuit system and the phase-frequencycharacteristic of the circuit. The capability of the circuit suppressingpower frequency common mode interference is getting improved, whichcontributes to the improvement of the quality of the sampledbioelectrical signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are schematic diagrams of the right leg drivingcircuit and the equivalent circuit thereof in the known ECG measuringsystem respectively;

FIG. 2 is a schematic diagram of the actual equivalent circuit of theknown right leg driving circuit;

FIG. 3 shows one embodiment of the known right leg driving circuit;

FIG. 4 is a frequency characteristic graph of the embodiment of theknown right leg driving circuit;

FIG. 5 is a block diagram of the improved right leg driving circuit forsuppressing power frequency common mode interference according to thepresent invention;

FIG. 6 is a concrete schematic diagram of the improved circuit accordingto the present invention; and

FIG. 7 is a flow chart of the method for suppressing power frequencycommon mode interference according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiments of this invention will bedescribed in detail in conjunction with the figures.

FIG. 5 is a block diagram showing the principle of the improved (forexample an ECG detection) driving circuit (for example right leg). Acommon signal extracted from a bioelectric signal (for example an ECGsignal) by a common mode interference signal extracting circuit (knownin the art and not shown) is provided into the right leg drivingcircuit, by which the phase of the common signal is changed and thenamplified. The right leg driving circuit may output two-way amplifiedsignals, in which one amplified signal which can be regarded as afeedback signal is selectively outputted to a living body onexamination. In FIG. 5, phase compensating and processing means isprovided for receiving both a bioelectrical signal from the living bodyon examination and the other amplified signal outputted from the drivingcircuit, determining a phase compensation amount of the other amplifiedsignal outputted from the driving circuit according to a characteristicvalue of the power frequency interference in the bioelectrical signal soas to phase-compensate the other amplified signal outputted from thedriving circuit, and selectively outputting the phase-compensatedamplified signal to the living body on examination.

The phase compensating and processing means includes at least one A/Dconverter (ADC) for converting the bioelectrical signal and the otheramplified signal outputted from the driving circuit into digital signalsrespectively and supplying the digital signals to a microprocessor,which determines the phase compensation amount of the other amplifiedsignal outputted from the driving circuit according to thecharacteristic value of the power frequency interference in thebioelectrical signal; and a D/A converter (DAC) for receiving a signalfrom the microprocessor and converting the signal into an analog signalas an output of the phase compensating and processing means. The signaloutputted by the phase compensating and processing means may be filteredvia a low-pass filter (LPF) and then transmitted to the living body onexamination. The A/D converter and D/A converter may be integrated intothe microprocessor.

The improved circuit can also include switching and selecting means withtwo selecting terminals, one of which is connected to an output terminalof the driving circuit, the other of which is connected to an outputterminal of the phase compensating and processing means, and theswitching and selecting means selectively outputs the amplified signaloutputted by the driving circuit and the phase-compensated amplifiedsignal outputted by the phase compensating and processing means to theliving body on examination (for example but not limited to the rightleg). The switching and selecting means may select and output receivedsignals by manual operation (for example a slide switch), or by thecontrol of the system software through connecting a control terminal ofthe switching and selecting means to the microprocessor.

FIG. 6 is a concrete schematic diagram of the above block diagram. Theright leg driving circuit includes an inverting amplifier which ismainly composed of an amplifier integrated circuit U1 (for example butnot limited to LM358), the gain of which can be set about −40 times (50Hz). A capacitor C1 is used for decreasing high-frequency negativefeedback gain, preventing high frequency self-excitation, andmaintaining the stability of the feedback loop. A resistor R4 connectedto an output terminal of the amplifier integrated circuit U1 is used forlimiting current to ensure the amount of leakage current no more than 50uA under a single failure mode. At the same time, the resistor R4 isalso used for low-pass filtering together with a capacitor C2 which isconnected to the other end of the resistor R4, so as to maintain thestability of the loop. The A/D converter may employ 8-bit or 12-bitanalog-to-digital converter of which the sampling rate is not less than1 kHz, for example but not limited to MAX1290. The D/A converter maycorrespond to the A/D converter, for example but not limited to MX7545A.The microprocessor fulfills the function of controlling and dataprocessing, for example but not limited to MCS-51 family of one chipmicroprocessor or a CPU and the like.

The low-pass filter is mainly used for filtering out high frequencysignals which is caused by the D/A conversion, and it may be achievedbut not limited to a simple one-order RC filtering circuit, as shown bya resistor R5 and a capacitor C3 in the figure. The cutoff frequency ofthe filter is slightly less than a half of the sampling frequency.Filter circuits in other forms will not be further discussed here, sincethey are well-known in the art. The switching and selecting means inthis embodiment may employ but not limited to MC14053 to be controlledby the microprocessor.

Since there is a phase leading between the phase shift of the originaloutput signal of the driving circuit and the phase shift of 180°required by the negative feedback system, the signal can bephase-compensated by means of delay output. Therefore the method of thepresent invention for suppressing power frequency common modeinterference may adopt follow steps based upon the above-mentionedhardware circuit or its equivalently transfer circuits:

A. providing phase compensating and processing means between the drivingcircuit and a living body on examination;

B. receiving an amplified signal outputted from the driving circuit anda bioelectrical signal from the living body on examination by the phasecompensating and processing means;

C. analyzing the characteristic of the bioelectrical signal anddetermining a phase compensation amount of the amplified signaloutputted from the driving circuit by the phase compensating andprocessing means, wherein the characteristic of the bioelectrical signalis represented by a characteristic value of the power frequencyinterference in the bioelectrical signal;

D. performing a corresponding time delay processing on the amplifiedsignal outputted by the driving circuit;

E. selectively transmitting the delay signal to the living body onexamination.

Assume the leading phase of the feedback signal outputted by the rightleg driving circuit of the measuring system is θ, the system samplingfrequency is f_(s), then the time t for which the output of the drivingsignal needs to be delayed is:

$\begin{matrix}{t = \frac{\theta}{2\pi\; f_{s}}} & ( {4\text{-}1} )\end{matrix}$

Since the signal outputted by the D/A conversion is a discrete signal,the real time for which the output is delayed is an integral multiple ofthe sampling period:

$\begin{matrix}{{t_{n} = \frac{n}{f_{s}}},{n = 0},1,{2\mspace{14mu}\ldots}} & ( {4\text{-}2} )\end{matrix}$

Then, the corresponding delayed phase of the signal is:

$\begin{matrix}{{{\Delta\theta}_{n} = {{2\pi\;\frac{t_{n}}{T}} = {100\pi\frac{n}{f_{s}}}}},{n = 0},1,{2\mspace{14mu}\ldots}} & ( {4\text{-}3} )\end{matrix}$

The compensation algorithm does not always make Δθ_(n)=θ, but can letΔθ_(n) approximately equal to θ, thus the leading phase of the systemcan be compensated to a certain extent, and the better signal qualitycan be achieved. At the same time, it can be seen that the higher thesampling frequency is, the more Δθ_(n) approximates to θ and the betterthe compensation effect is.

Since the right leg driving circuit and the added phase compensating andprocessing means must constitute a negative feedback system, the leadingphase of the original driving signal has a interval of [0, π/2). It istestified by experiments that the intensity of the power frequencyinterference signal has a unique minimum value when the driving signalis phase-compensated in this interval. Therefore, the optimum timet_(opt) of the delay output can be found by use of the trend judgment,more specifically, by increasing the delay-output time, if the intensityof the power frequency interference decreases, it is recognized that thecompensation is in a proper direction and then the delay time can befurther increased. On the contrary, the delay time is considered asexceeding the optimum delay time, and the previous delay time can beregarded as the optimum delay time t_(opt). In addition, the maximaldelay time can be regarded as the optimum delay time t_(opt), if theintensity of the power frequency interference signals continuouslydecreases in the interval of [0, π/2).

In the compensating phase interval of [0, π/2), n is an integer chosenfrom [0, K], in which

${K = {{{int}( \frac{f_{s}}{200} )} - 1}},$and int( ) denotes rounding operation. We call n as the system statevalue. The delay time of the output signal of the driving circuitcorresponding to the system state value n is set as t_(n). The intensityof the power frequency interference signals in the bioelectrical signalsis represented with a characteristic value F_(n). This characteristicvalue F_(n) is defined as a sum of peak to peak values of the extractedpower frequency interference signals within a plurality periods ofinterference signals. Thus, the step C includes following cycleprocessing procedures (as shown in FIG. 7):

C1) initializing a system state value of the measuring system;

C2) extracting a characteristic value of the power frequencyinterference in a current system state of the measuring system;

C3) adding 1 to the system state value and extracting anothercharacteristic value again;

C4) if the another characteristic value being decreased, it indicatesthe power frequency interference is reduced, thus the direction of thecompensation is correct, then continuing the step C3) till the systemstate value becomes maximized; otherwise, it indicates the direction ofthe compensation is incorrect, then subtracting 1 from the system statevalue, and then an optimum system state value can be selected, whereinthe system state value is integer and corresponds to one phasecompensation state;

C5) setting the optimum system state value in the step 4) as a statevalue of the current system, to determine a corresponding phasecompensation amount; the above steps C1)˜C5) can be performedcircularly.

In FIG. 7, for the sake of clearness, a variable F_(ea) is used forstoring the characteristic value in the current system state. Anothercharacteristic value after the system entering into a next state will becompared with the variable to determine which one is greater. If Jdenotes a system optimum state value, the intensity of the powerfrequency interference signals in this optimum state is a local minimumvalue, and the time for delay output corresponding to the optimum stateis

$t_{opt} = {\frac{J}{f_{s}}.}$

Take the ECG signal measuring system with sampling frequency f_(s)=1 kHzas an example,

${K = {{{{int}( \frac{f_{s}}{200} )} - 1} = 4}},$then n is chosen from [0, 4]. There are five compensating states of theright leg driving circuit within the phase compensation interval of [0,π/2). When n=0, 1, 2, 3, 4, the delay-output time of the right legdriving signal is 0, 1 ms, 2 ms, 3 ms, 4 ms respectively, then thecorresponding phase compensation amounts are

$0,{\frac{1}{10}\pi},{\frac{1}{5}\pi},{\frac{3}{10}\pi},{\frac{2}{5}{\pi.}}$If the characteristic value of the power frequency interference isminimal when n=2, then this state will be regarded as the system optimumstate, thus the delay time at this point is set to 2 ms.

The phase compensating and processing means receives the bioelectricalsignal in the step B. The sampling can be performed by the A/D converterunder the control of the microprocessor, and then a characteristicanalysis on sampled data is performed in step C. In the characteristicanalysis, the power frequency interference characteristic value may becalculated by following steps of:

a) successively storing data of the bioelectrical signal sampled by theA/D converter into a predetermined data array;

b) receiving the data array by a digital band-pass filter so as toextract the power frequency interference signal and output related data;

c) detecting a maximum value and a minimum value of the output datawithin a period, so as to calculate the peak to peak value of the signalwithin the period;

d) calculating a sum of the peak to peak values in a plurality ofadjacent periods so as to get the characteristic value F_(n) of thepower frequency interference.

The band-pass filter may be a simple band-pass filter, for example butnot limited to, two-order Butterworth band-pass filter with a centerfrequency of 50 Hz or 60 Hz and a bandwidth of ±2 Hz. It will be notfurther discussed here, since it is well-known in prior art.

Correspondingly, the step D for performing a corresponding time delayprocessing on the amplified signal outputted by the driving circuitincludes steps of:

D1) controlling the A/D converter by a microprocessor of the phasecompensating and processing means, setting a sampling channel and asampling frequency f_(s), and sampling the amplified signal outputted bythe driving circuit;

D2) creating a data array org_data[K+1], and successively storingamplified signal sampled by the A/D converter;

D3) determining a delay-output datum according to an optimum state valueJ of the current system, which corresponds an array elementorg_data[K−J] of the data array;

D4) converting the datum outputted at the sampling frequency f_(s) intoan analog signal by the D/A converter and then outputting the analogsignal.

The successively storing means, after each time of A/D conversion, eachdata originally stored in the array element is stored in another arrayelement located just before the array element, while the array elementorg_data[K] stores the just sampled data. Other storing method which isequivalent with or transformed from the above manner may be adopted.Further description will be omitted.

The step D in which the signals outputted by the driving circuit isreceived and a corresponding time delay processing is performed on thesignals may employ other processing methods. For example, the drivingsignal can be output directly via a delayer which is controlled by themicroprocessor and the delay time thereof is adjustable, which alsofalls into the scope of this invention.

The embodiments of this invention is testified by the experiments ofhuman body ECG measurement that the intensity of power frequencyinterference signals in original ECG signals can be reduced by more thana half once the sampling frequency f_(s) is 1 kHz. It is indicated fromthe above analysis that the more accurate delay time and better effectfor suppressing power frequency interference can be achieved if highersampling frequency is adopted.

1. An apparatus for suppressing power frequency common mode interferencein a bioelectrical signal measuring system, comprising: a drivingcircuit configured to amplify and change the phase of a common modeinterference signal to produce a first driving signal, wherein thecommon mode interference signal is extracted from a plurality of firstelectrodes attached to a patient; phase compensating and processingcircuitry electrically connected to the driving circuit for receivingthe first driving signal, the phase compensating and processingcircuitry configured to produce a second driving signal byphase-compensating the first driving signal based on a characteristicvalue of power frequency interference in a bioelectrical signal acquiredthrough the plurality of first electrodes; and a switch to receive thefirst driving signal and the second driving signal, the switchconfigured to selectively switch between providing the first drivingsignal and providing the second driving signal to a second electrodeattached to the patient.
 2. The apparatus of claim 1, wherein the phasecompensating and processing circuitry comprises: at least oneanalog-to-digital (A/D) converter to convert the bioelectrical signaland the first driving signal from the driving circuit into respectivedigital signals; a microprocessor to receive the digital signals and todetermine a phase compensation amount of the first driving signalaccording to the characteristic value of the power frequencyinterference in the bioelectrical signal; and a digital to analog (D/A)converter for receiving a signal from the microprocessor and convertingthe signal into an analog signal.
 3. The apparatus of claim 1, furthercomprising: a low-pass filter to filter the phase-compensated seconddriving signal.
 4. A method for suppressing power frequency common modeinterference in a bioelectrical signal measuring system, comprising:receiving a common mode interference signal extracted from a pluralityof first electrodes attached to a patient; amplifying and changing thephase of the common mode interference signal to produce a first drivingsignal; producing a second driving signal by phase-compensating thefirst driving signal based on a characteristic value of power frequencyinterference in a bioelectrical signal acquired through the plurality offirst electrodes; and selectively switching between providing the firstdriving signal and providing the second driving signal to a secondelectrode attached to the patient.
 5. The method of claim 4, whereinphase-compensating the first driving signal comprises: initializing asystem state value of a measuring system; extracting a firstcharacteristic value of the power frequency interference in a currentsystem state of the measuring system; adding 1 to the system state valueand extracting a second characteristic value; if the secondcharacteristic value has decreased from the first characteristic value,determining that the power frequency interference is reduced and thatthe direction of the compensation is correct, and continuing to step upthe system state value and evaluating corresponding characteristicvalues until the system state value becomes maximized; if the secondcharacteristic value has increased from the first characteristic value,determining that the direction of the compensation is incorrect,subtracting 1 from the system state value, and selecting an optimumsystem state value, wherein the system state value is an integer andcorresponds to one phase compensation state; and setting the optimumsystem state value as a state value of the current system to determine acorresponding phase compensation amount.
 6. The method of claim 5,wherein the system state value ranges from [0, K], in which${K = {{{int}( \frac{f_{s}}{200} )} - 1}},$ int( ) denotes arounding operation, and f_(s) denotes a sampling frequency of thebioelectrical signal.
 7. The method of claim 5, wherein thecharacteristic value of the power frequency interference refers to a sumof peak to peak values of the power frequency interference signalextracted from the bioelectric signal within a plurality of periods,which is calculated by: successively storing data of the bioelectricalsignal sampled by an ND converter into a predetermined data array;receiving the data array by a digital band-pass filter so as to extractthe power frequency interference signal and output related data;detecting a maximum value and a minimum value of the output data withina period so as to calculate the peak to peak value of the signal withinthe period; and calculating a sum of the peak to peak values in aplurality of adjacent periods so as to determine the characteristicvalue of the power frequency interference.
 8. The method of claim 4,further comprising performing a corresponding time delay processing onfirst driving signal by: controlling an A/D converter using amicroprocessor to set a sampling channel and a sampling frequency f_(s),and to sample the first driving signal outputted by the driving circuit;creating a data array org_data[K+1], and successively storing anamplified signal sampled by the A/D converter; determining adelay-output datum according to an optimum state value J of the currentsystem, which corresponds an array element org_data[K−J] of the dataarray; and converting the datum outputted at the sampling frequencyf_(s) into an analog signal by an D/A converter; wherein${K = {{{int}( \frac{f_{s}}{200} )} - 1}},$ int( ) denotes arounding operation.
 9. The method of claim 4, further comprisingproviding the first driving signal outputted by the driving circuit to adelayer for performing a corresponding time delay processing, which iscontrolled by a microprocessor and whose delay time is adjustable. 10.An apparatus for suppressing power frequency common mode interference ina bioelectrical signal measuring system, comprising: driving means foramplifying and changing the phase of a common mode interference signalto produce a first driving signal, wherein the common mode interferencesignal is extracted from a plurality of first electrodes attached to apatient; phase compensating and processing means electrically connectedto the driving means for receiving the first driving signal, the phasecompensating and processing means for producing a second driving signalby phase-compensating the first driving signal based on a characteristicvalue of power frequency interference in a bioelectrical signal acquiredthrough the plurality of first electrodes; and switching means forreceiving the first driving signal and the second driving signal, theswitch means for selectively switching between providing the firstdriving signal and providing the second driving signal to a secondelectrode attached to the patient.
 11. The apparatus of claim 10,wherein the phase compensating and processing means comprises: firstconverting means for converting the bioelectrical signal and the firstdriving signal from the driving means into respective digital signals;processing means for receiving the digital signals and to determine aphase compensation amount of the first driving signal according to thecharacteristic value of the power frequency interference in thebioelectrical signal; and second converting means for receiving a signalfrom the microprocessor and converting the signal into an analog signal.12. The apparatus of claim 10, further comprising: filtering means forfiltering the phase-compensated second driving signal.